Cellular telephony and other forms of radio based communication are becoming increasingly popular. As the radio frequency spectrum becomes more crowded and heavily used, the various regulatory agencies have found it necessary to impose tight restrictions on the RF emissions of such devices. In conjunction with certifying that the device complies with regulations, the manufacturer may need to operate it over a wide range of conditions and verify that such measurable quantities as power level and frequency are within specified limits. For example, for both regulatory compliance and customer related quality assurance reasons, exhaustive testing of an extensive collection of such operations might be performed at the factory before the unit is shipped. Additional manufacturing concerns include trouble shooting during assembly and routine Quality Assurance checks.
Regardless of whether "lab bench" type test equipment or production line oriented "automated test equipment" is used by a manufacturer to perform such measurements, it is quite likely that some sort of fixture is necessary to connect the test equipment to the item to be tested. Such a fixture needs to be easy to use and produce repeatable results. It should not unduly affect the measured results, and any effects it does produce should be "removable" by compensation or application of calibration constants. It would also be desirable if such a fixture were durable and relatively inexpensive.
Once a unit has been sold it may later need service. The unit might or might not be completely inoperative, and the owner or user of the unit may wish to bring it to a location for servicing that might, for example, be the retail location where the unit was originally purchased. The idea is that the unit can be tested by minimally trained personnel at the retail establishment to ascertain if at least its major functions are operational. An automated test set can be used to perform these tests, and its level of sophistication may range from basic and fundamental tests common to all brands of devices meeting a particular class of service, to exhaustive factory level verification for a specific model from a particular manufacturer. Economic considerations and the level of skill sometimes needed to configure the tester and interpret results are a factor in test set design.
Such a test set may be used at a retail establishment to verify proper operation: (a) upon initial sale; (b) upon initial configuration or reconfiguration of configurable properties within the unit; (c) subsequent to a repair; and, perhaps most importantly (d) assist in deciding if a unit that has been in for service is in need of repair.
Thus, manufacturers, retail sales outlets and field service organizations all have a need for a fixture that will couple a wireless telephone, radio or other RF device to various types of test equipment.
Let us refer to the wireless telephone, radio or other RF device to be tested as the DUT (Device Under Test). The DUT will sometimes have a jack for an external antenna which, when a suitable plug is inserted therein, will disconnect the antenna from the DUT and connect in its place whatever is connected to the plug. There are at least three reasons why the use of such an external antenna jack may not be suitable for exhaustive factory level testing, service center (depot) testing, or cursory trouble shooting and testing performed by relatively untrained personnel at retail establishments. First, there may be more than just the antenna that is disconnected; it might be an antenna and a matching network, so that as far as the DUT is concerned the test set-up is a noticeably different electrical arrangement requiring informed interpretation for proper understanding. Second, the manufacturer may expect that such a connector will be seldom used anyway, and be prone to avoid the expense of utilizing a truly robust connector. The switching function of the connector located in the "blow hole" of some brands cell phone have been observed to fail after a mere half-dozen or so uses. Third, there is the mundane issue of having on hand the proper plug to fit the jack. Some manufacturers are fond of truly odd or proprietary designs, which may be hard to find or, worse, are no longer supplied.
A desirable alternative to this situation is to couple RF energy directly from the antenna of a DUT into the measuring test equipment or other test set (hereinafter called the measuring equipment). This has the further advantage of testing the thing of interest "as is", so to speak, without introducing any extraneous circumstances that complicate the issue. This implies the use of some sort of coupler between the antenna of the DUT and the measuring equipment. In principle, the same coupler can be used for tests that involve applying RF energy from a source in the measuring equipment to the DUT.
Such a coupler should have a number of properties. It should be flat; that is have a relatively constant amount of attenuation over the frequency band or bands of interest. For wireless phones in the U.S. this might be more than one band, say, 825-895 MHZ ("cellular") and 1.850-1.990 GHz (PCS). It is even better if two such bands (as found in dual mode phones) can be construed as a single band (0.825-1.99 GHz). Either way, the amount of attenuation should be small, say on the order of three to six dB. The reason for this latter requirement is that some specifications for cell phones are really quite rigorous. The specification for power measurement may be .+-.2/10 dB at -55 dBm for a carrier at 1.8 GHz. If the coupler has a 20 dB of attenuation, then even a top of the line laboratory spectrum analyzer (or power meter) may be unable to resolve to 2/10 dB at such low levels, owing to its internal noise floor. Finally, considering the frequencies involved (for wireless phones, anyway) the coupler ought to not touch the antenna, but rely on true electromagnetic coupling instead of ohmic contact. The reasons for this are readily understood when it is appreciated that: (a) The antenna is often covered with a nonconductive sheath or coating; (b) Such a mechanical contact inserts an unwanted impedance into the measurement setup; and (c) Such a mechanical contact is apt to be unreliable and of questionable repeatability.
One prior art coupler manufactured by Wavetek (Triangle Park, N.C.). It consists of a tablet within which are embedded several patch antennae. A clamp hold the DUT in a fixed location. It appears to have about 26 dB of loss, and also leaves something to be desired with respect to repeatablilty.
Another prior art coupler design, manufactured by ETS (P.O. Box 80589, Austin, Tex.), for a wireless phone incorporates a single patch antenna of empirically determined shape into a shielded anechoic box. Mechanical fixturing holds the phone in a predetermined position relative to the patch antenna, but different coupling coefficients obtain for each model having a different case.
Thus it would be desirable if there were an affordable and technically adequate antenna driven coupler for radios and wireless phones that offered high bandwidth, flat response and low loss.